Transdermal administration of drugs is increasing in popularity. Most transdermal systems deliver the drug by passive diffusion using the concentration gradient as the driving force. Facilitated transdermal systems are also available and include penetration enhancers, phonophoresis and iontophoresis. Most transdermal systems use the non-ionized form of the drug with the exception of iontophoresis, which uses the ionized form. Upon application of a small electric current and the placement of the charged drug under the appropriate electrode, the drug is "repelled" away from the electrode through the skin.
There are four problems typically associated with drug delivery via iontophoresis. First, as the current is applied, electroendosmotic (EEO) flow occurs wherein water moves from the drug-containing matrix into the skin. This osmotic flow carries drug with it but also causes water to accumulate in the interface of the patch and the skin, and limits the time the patch is usable. Second, an ionized drug in aqueous solution is not as stable as one in a non-aqueous system and tends to degrade more rapidly. Most of the patch matrices currently in use include the drug in a hydrophilic "gel", which contains a significant percentage of water. A third problem is that such gels tend to dry out with time, rendering the patch ineffective. Fourth, most iontophoretic patches last for only a few hours under active usage. In most iontophoretic transdermal delivery systems the drug is placed in a solution which is placed on a gauze pad or instilled into a special electrode-containing reservoir with a gel barrier or is incorporated into a water-based hydrogel. An iontophoretic system which employed a drug matrix which avoided most of these problems would be desirable.